1) Field of the Invention
The present invention relates to a mobile terminal and a method of controlling reception of the same, and relates to a technique suitably applied in a system using, for example, an HSDPA (Hi-Speed Downlink Packet Access) transmission system that is one transmission system in the mobile communication system.
2) Description of the Related Art
A W-CDMA (Wideband-Code Division Multiple Access) system is a 3rd generation mobile communication system, and is being standardized by the 3GPP (3rd Generation Partnership Project). The HSDPA is provided as a system for realizing the maximum transmission speed of 14 Mbps through a downlink, as one of the themes of the standardization.
The HSDPA uses the adaptive coding modulation procedure. One feature of the HSDPA is to adaptively switch between, for example, the QPSK (Quadrature Phase Shift Keying) modulation procedure and the 16QAM (Quadrature Amplitude Modulation) system in accordance with the wireless environment between a base station and a mobile terminal device (hereinafter referred to also as a mobile station).
The HSDPA uses an H-ARQ (Hybrid Automatic Repeat reQuest) system. In the H-ARQ, when the mobile station detects an error in data received from the base station, the data is retransmitted from the base station in response to a request from the mobile station. Then, the mobile station performs error correction decoding using both of the already-received data and the retransmitted data. In this manner, in the H-ARQ, the already-received data is effectively used even if an error occurs, thereby enhancing the gain of the error correction decoding and reducing the number of retransmission.
The HSDPA uses mainly the following wireless channels of HS-SCCH (High Speed-Shared Control Channel) HS-DSCH (High Speed-Downlink Shared Channel), HS-DPCCH (High Speed-Dedicated Physical Control Channel).
Both of the HS-SCCH and the HS-DSCH are downlink (i.e. in a direction from the base station to the mobile station) shared channels. The HS-SCCH is a control channel for sending various parameters (L1 information) regarding data to be sent through the HS-DSCH. In this case, various parameters includes modulation type information (representing with which modulation procedure data is sent through the HS-DSCH), an assigned number of a diffusion code (code multiplexing number) a process number of HS-DSCH, a retransmission/new indicator representing whether transmission data is to be retransmitted and a pattern of rate matching to be performed for the transmission data.
In the HS-SCCH, control signals can concurrently be sent to a plurality of mobile stations using a plurality of diffusion codes (e.g. four codes). Of the plurality of HS-SCCHs, the mobile station can determine the HS-SCCH that is addressed thereto, based on a UE-ID (User Equipment-Identity).
On the other hand, the HS-DPCCH is an individual control channel in the uplink direction from the mobile station to the base station, and is used when the mobile station sends an ACK or NACK signal to the base station in accordance with whether data received through the HS-DSCH can be received or not. When the mobile station fails to receive data (i.e. when a CRC (Cyclic Redundancy Check) error is found in the received data), the base station controls a retransmission process in response to a NACK signal sent from the mobile station.
Further, the HS-DPCCH is used when the mobile station sends a measurement result of reception quality (e.g. an SIR: Signal to Interference Ratio) of a received signal from the base station, to the base station as a CQI (Channel Quality Indicator). The base station determines whether the downlink wireless environment is favorable based on the received CQI. If it is determined that the downlink wireless environment is favorable, the modulation procedure is switched to one capable of transmitting data at a high speed. On the contrary, if it is determined that the downlink wireless environment is not favorable, the modulation procedure is switched to a procedure for transmitting data at a low speed (i.e. adaptive modulation is performed).
In the HSDPA, before and after a rate matching process is performed, a ratio of a small amount of HS-DSCH data becomes lower than that of a large amount of HS-DSCH data, thus possibly realizing favorable transmission characteristics.
Note that the term “ratio before and after the rate matching” relates to a rate matching process on the sender side (base station), as exemplarily shown in FIG. 9, and can be expressed by the relationship of the (ratio before and after the rate matching)=(information bit length)÷(bit length after the rate matching). The lower the value of the “ratio before and after the rate matching” is, the higher the ratio of the transmission power per bit becomes, because redundancy is introduced to a small amount of information so as to become transmission data of a large bit length, thus realizing favorable transmission characteristics.
On the other hand, data of the HS-SCCH has a constant number of information bits at the time of data transmission. In the communication environment of a low SIR, favorable transmission characteristics are not realized, even if the above transmission control is performed based on the CQI.
Japanese Patent Application Laid-Open No. 2004-312530 discloses a technique for improving the transmission power for the HS-SCCH, when the reception of the HS-SCCH is deteriorated.
Japanese Patent Application Laid-Open No. 2005-318470 discloses a technique for the transmission control on the side of the mobile station. According to this technique, a BLER of the HS-DSCH becomes constant by adding an offset to the CQI sent from the mobile station for the sake of modulation.
Further, Japanese Patent Application Laid-Open No. 2002-369258 discloses a technique for efficiently sending control information from the mobile station to the base station.
3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Layer procedures (FDD) (Release 7) (3GPP TS25.214V7.0.0 (2006-03)) discloses the above-described conventional technique.
In the HSDPA reception, the HS-SCCH is received, and the parameters are extracted therefrom. Then, the HS-DSCH is received. Depending on the transmission environment and different coding systems between the HS-SCCH/HS-DSCH, the HS-SCCH may not be received, while the HS-DSCH may be received. In such an environment, the HS-DSCH may fail to be received.
The below describes the problems and related techniques.
[1] Channel Configuration and its Timing Relationship in HSDPA
FIG. 4 is a diagram showing the channel configuration and its timing relationship in the HSDPA. Because the W-CDMA uses a code division multiplex system, each channel is divided by code. Though not illustrated, the mobile station uses a CPICH (Common Pilot Channel) as a channel for sending a so-called pilot signal. This channel is used for channel estimation, cell search and timing reference for another down physical channel with in the same cell, in the mobile station. In addition, the mobile station uses a P-CCPCH (Primary Common Control Physical Channel) as a channel for sending report information.
As shown in FIG. 4, in each channel, one frame (10 ms) is composed of fifteen slots.
The frame head of the HS-DSCH delays by two slots as compared to the HS-SCCH. This delay enables the mobile station to demodulate the HS-DSCH using a demodulation procedure corresponding to a modulation procedure represented in the received information, after it receives the information through the HS-SCCH. Each of the HS-SCCH and HS-DSCH includes one sub-frame composed of three slots.
The HS-DPCCH is a channel of an uplink direction. The first slot of the HS-DPCCH is used for sending an ACK/NACK signal representing a result of the received HS-DSCH from the mobile station to the base station approximately after 7.5 slots since the reception of the HS-DSCH. The second and third slots are used for sending CQI information for controlling the adaptive modulation periodically to the base station for the sake of feedback transmission. In this case, the CQI information to be sent is generated based on the receiver environment (e.g. a result of measured SIR) that is measured in a period that is four to one slot prior to the CQI transmission.
[2] Data Contents to be Sent Through HS-SCCH and Its Encoding Procedure
The following (1) to (7) are examples of data to be sent through the HS-SCCH. Each of the following data is used for a receiving process for corresponding HS-DSCH (delay of two slots).
(1) Xccs (Channelization Code Set information)
(2) Xms (Modulation Scheme information)
(3) Xtbs (Transport Block Size information)
(4) Xhap (Hybrid ARQ Process information)
(5) Xrv (Redundancy and constellation Version)
(6) Xnd (New Data indicator)
(7) Xue (User Equipment identify)
Note that (1) “Xccs” represents a diffusion code for sending data through the HS-DSCH (e.g. data representing a combination of a multicode number and a code offset), and is formed of seven bits.
(2) Xms represents whether the modulation procedure of the HS-DSCH is either QPSK or 16QAM, and is formed of one bit.
(3) Xtbs is used for calculating a transport block size of data to be transmitted through the HS-DSCH (i.e. the size of data to be sent in one sub-frame of the HS-DSCH), and is formed of six bits.
(4) Xhap represents a process number of H-ARQ, and is formed of three bits.
The base station cannot determine whether already-transmitted data can be received by the mobile station until an ACK or NACK signal is received. However, the base station sends a new packet before an ACK or NACK signal is received, because the transmission efficiency decreases if it waits for the ACK or NACK signal.
Since the mobile station uses the H-ARQ system, it needs to identify with which received data retransmission data should be synthesized. Process numbers (e.g. 0, 1, . . . , 7 up to 8) are defined for the respective data to be sent in the sub-frames. When the same process number exists, the corresponding HS-DSCH data can be synthesized. It takes at least 6TTI (Transmission Time Interval) (1TTI=2 ms in this case) all the way through since the mobile station performs a CRC process upon reception of the previous HS-DSCH data, sends a NACK signal to the base station, and receives retransmission data from the base station. Therefore, the retransmission data should not be synthesized in a period less than 6TTI.
(5) Xrv represents a rate matching patter and a type of constellation rearrangement in the retransmission of the HS-DSCH, and is formed of three bits.
(6) Xnd represents whether an HS-DSCH transmission block is a new block or a retransmission block, and is formed of one bit. If the HS-DSCH transmission block is a new block, the above H-ARQ process is not performed.
(7) Xue represents identification information of the mobile station, and is formed of sixteen bits.
[3] Encoding (Decoding) Process
FIG. 5 is a diagram exemplarily showing the above data flow. In FIG. 5, a numeral 100 identifies a base station (BTS: Base Transceiver Station), while a numeral 200 identifies a mobile terminal (MS: Mobile station) Note that the mobile terminal 200 may be referred simply as a mobile station 200.
As shown in FIG. 5, the base station 100 mainly includes, for example, a CRC adding unit 101, a turbo-encoder 102, a 1st rate-matching unit 103, a virtual IR (Virtual Incremental Redundancy) buffer 104, a 2nd rate-matching unit 105 and a physical CH (channel) mapping unit 106.
In thus configured base station 100, transmission data addressed to the mobile station 200 is input to the CRC adding unit 101, and the unit 101 adds a CRC code for a CRC process thereto. Then, the transmission data is input to the turbo-encoder 102, and the turbo-encoder 102 performs an error correction encoding process (a turbo-encoding process in this case) therefor.
For the turbo-encoded transmission data, the 1st rate-matching unit 103 performs a puncture (thinning) process for deleting a predetermined bit(s) using a predetermined algorithm or a repetition process for repeating a predetermined bit(s). In this case, the data will be processed to a data amount suitable for a predetermined area of the virtual IR buffer 104 in the following stage. Then, the data is temporarily held in the virtual IR buffer 104 for timing adjustment.
After that, the 2nd rate-matching unit 105 performs the puncture or repetition process for the transmission data sent from the virtual IR buffer 104 in such a way that the data can be contained in one sub-frame of the physical channel that is a target channel for mapping (assignment) performed by the physical CH mapping unit 106 in the following stage. As a result, the data can be input to the physical CH mapping unit 106.
The physical CH mapping unit 106 maps the transmission data whose data amount has been adjusted by the 2nd rate-matching unit 105. Then, the transmission data is sent to the mobile station 200 through a non-illustrated transmission antenna.
The mobile station 200 mainly includes, for example, a physical CH (channel) separating unit 107, a 2nd rate-matching unit 108, a virtual IR buffer 109, a 1st rate-dematching unit 110, a turbo-decoding unit 111 and a CRC process unit 112.
In thus configured mobile station 200, a signal sent from the base station 100 is input to the physical CH separation unit 107 through an unshown reception antenna, and the physical CH separation unit 107 identifies and demaps the received physical channel so as to extract effective data, and the extracted data is input to the 2nd rate-dematching unit 108.
For the received data, the 2nd rate-dematching unit 108 performs a rate dematching process (for adjusting the data amount) that is an opposite process of the rate matching process (the above puncture or repetition process for adjusting the data amount) performed by the 2nd rate-matching unit 105 on the receiver side (the base station 100). The received data after the rate dematching process is temporarily held in the virtual IR buffer 109 for timing adjustment, and is input to the 1st rate-dematching unit 110.
For the received data from the virtual IR buffer 109, the 1st rate-dematching unit 110 performs a rate dematching process (for adjusting the data amount) that is an opposite process of the rate matching process (the above puncture or repetition process) performed by the 2nd rate-matching unit 105 on the sender side (the base station 100).
Thus rate-dematched, received data is turbo-decoded (error correction decoded) by the turbo-decoding unit 111, and is CRC processed by the CRC process unit 112. As described above, if the CRC result is “true” (OK), an ACK signal is sent to the base station 100. On the contrary, if the CRC result is “false” (NG), an NACK signal is sent thereto.
[4] Data Demodulation/Decoding Process
FIG. 6 is a diagram showing timings of receiving the HS-SCCH and HS-DSCH and various processing timings. For simplicity of description, FIG. 6 shows only data of the process No. 1. However, the same applies to other data. In FIG. 6, the process numbers of 0 to 5 are repeated.
For example, in the slot numbers #0 to #2 in the process No. 1, the mobile station 200 receives the HS-SCCH sent from the base station 100, extracts Xue included in the first half of the HS-SCCH, and determines whether this HS-SCCH is addressed to its own mobile station 200 in the slot number #1 in the process No. 1. If it is determined that the HS-SCCH is addressed to its own mobile station 200, the mobile station 200 extracts HS-DSCH demodulation parameters (such as a modulation procedure parameter, a code multiplexing number parameter, etc.) and HS-DSCH decoding parameters (such as a transport block size parameter, a rate matching parameter, etc.) from the last half of the received HS-SCCH. In the slot number #0 of the process No. 2, the mobile station 200 performs demodulation setting and decoding setting for the HS-DSCH that is received with a delay of two slots since the HS-SCCH is received.
If the HS-DSCH is received with a delay of two slots since the HS-SCCH begins to be received (i.e. from the slot number #2 of the process No. 1 to the slot number #1 of the process No. 2), the mobile station 200 performs a process for demodulating the received HS-DSCH in accordance with the HS-DSCH demodulation setting, in the slot number #0 of the new process No. 3. Then, the turbo-decoding unit 111 performs a process for decoding the received HS-DSCH, in accordance with the HS-DSCH decoding setting.
At this time, if the CRC result of the decoded HS-DSCH is “true”, as checked by the CRC process unit 112, the mobile station 200 sends an ACK signal to the base station 100. On the contrary, if the CRC result of the decoded HS-DSCH is not “true”, the mobile station 200 sends a NACK signal to the base station 100, and requests for retransmission of the HS-DSCH. After that, the mobile station 200 performs a process for synthesizing retransmission data from the base station 100 with the data held in the virtual IR buffer 109 (H-ARQ synthesizing).
[5] H-ARQ Synthesizing Process
FIG. 7A is a diagram for explaining an H-ARQ process when retransmission data is appropriately received. FIG. 7B is a diagram for explaining an H-ARQ process when retransmission data is not appropriately received. In this case also, the process numbers of 0 to 5 are repeated.
Description will now be made to the case where the mobile station 200 appropriately receives the retransmission data from the base station 100 as shown in FIG. 7A.
Upon reception of the HS-DSCH data of the process No. 0, the demodulation/decoding process and the CRC process is performed for the receive data in the mobile station 200. When the CRC process unit 112 determines that the CRC result is “false”, the mobile station 200 sends a NACK signal to the base station 100 through the HS-DPCCH at timing after 7.5 slots since the complete reception of the HS-DSCH (i.e. at a timing corresponding to the 1.5 slots of the process No. 3).
Upon reception of the NACK signal, the base station 100 retransmits transmission data (retransmission data of the process No. 0) whose rate matching pattern has been changed through the HS-DSCH. The mobile station 200 synthesizes the received data held in the virtual IR buffer 109 with the retransmission data at the same timing of the process No. 0 as that of the next cycle, and performs a turbo-decoding process therefor. Note that the H-ARQ synthesizing process is performed for the data received in the process No. 1, as illustrated with the dotted lines of FIG. 7A.
In this manner, in the mobile station 200 in the HSDPA, the received HS-DSCH data is synthesized with the retransmission data at a processing timing (cycle) of the same processing number. As a result, corresponding HS-DSCH data pieces can be synthesized without identifying which received data corresponds to the retransmission data using any special process. The base station 100 can send a next new packet before receiving the ACK or NACK signal from the base station 200, thus preventing a decrease in the transmission efficiency and realizing accurate synthesizing process.
[6] Problem at the Time of Receiving Data
Due to deterioration of a transmission environment between the base station 100 and the mobile station 200, the base station 100 may not appropriately receive the NACK signal, the base station 100 may not send retransmission data (retransmission data of the process No. 0 in this case), or the mobile station 200 may not receive the retransmission data.
As shown in FIG. 7B, it is assumed that the mobile station 200 can not receive the retransmission data of the process No. 0.
In this case, the mobile station 200 does not perform the synthesizing process at a timing corresponding to the process No. 0 in which the retransmission data should originally be synthesized. Instead, the mobile station 200 performs the synthesizing process at a timing corresponding to the same process No. 0 in which the retransmission data has appropriately been received. When a time period until retransmission data can appropriately be received is greater than a predetermined threshold value, the transmission data may not be subject to the synthesizing process.
In FIG. 7B, because the retransmission data can appropriately be received, data corresponding to the process No. 1 can be synthesized with the retransmission data at the timing of the process No. 1 of the next cycle, as in the case of FIG. 7A.
With reference to FIG. 8, description will now be made to control processing for a transmission using CQI (s). FIG. 8 shows a CQI mapping table which holds CQIs in association with transport block size parameters, code multiplexing number (1 to 5) parameters and modulation procedure (QPSK, 16-QAM) parameters.
The base station 100 changes the format of the HS-DSCH so as to make the format correspond to a transmission parameter corresponding to the CQI value stored in the CQI mapping table, and sends the data. In regard to the relationship between the CQI values and the total amount of data, the lower the CQI value is, the smaller the total amount of data becomes. In addition, the higher the CQI value is, the lager the total amount of data becomes.
When the base station 100 sends data through the HS-SCCH or DSCH, the transmission power value is always constant. However, depending on the positional relationship (distance, etc.) between the base station 100 and the mobile station 200, different apparent transmission power values are identified by a plurality of mobile stations 200. An SIR is generally used as an indicator indicating the transmission power from the base station 100 identified at the mobile station 200. For example, when the mobile station 200 exists near the base station 100, the SIR is high. On the contrary, when the mobile station 200 exists far from the base station 100, the SIR is low.
To improve the communication efficiency, the mobile station 200 sends a high CQI value to the base station 100, when a high SIR is identified. As a result, a large amount of transmission data is sent from the base station 100. On the contrary, when a low SIR is identified, the mobile station 200 sends a low CQI value to the base station 100. As a result, a small amount of transmission data is sent from the base station 100, thus surely sending/receiving the data.
That is, in the context of the same transmission power value (SIR), the lower the CQI value is, the lesser the error occurs (BLER (Block Error Rate) is improved (i.e. low)). On the contrary, the higher the CQI value is, the more the error occurs (the BLER is high).
In consideration of the above, according to the basic specifications of the 3GPP, it is defined that CQI values are sent to the base station 100 in the Static environment (where there is no fading). In this case, specifically, a CQI value corresponds to the transport block size, the code multiplexing number and the modulation procedure. This CQI value is so selected that the BLER of the HS-DSCH measured by the mobile station 200 is not greater than 10% (i.e. BLER<0.1).
Further, there is such a relationship as shown in the following equation (1) between the transmission power value (PHS-DSCH) of the HS-DSCH and the transmission power value (PCPICH) of the CPICH.PHS-DSCH=PCPICH+Γ (Γ: a fixed value specified in high-order)  (1)
As seen from this equation (1), there is a correlation between the power value of the HS-DSCH and the power value of the CPICH. When the SIR of the CPICH is high, both of the quality of the HS-DSCH and the BLER are improved. On the contrary, when the SIR of the CPICH is low, the BLER of the HS-DSCH is degraded.
According to an actual transmission parameter controlling method using the CQI values, such an SIR that the BLER is equal to 0.1 (BLER=0.1) in the static environment is searched in advance, based on each transmission parameter in association with each CQI value of the CQI mapping table (the transport block size parameter, the code multiplexing number, the modulation procedure, etc.). The CQI value and the searched an SIR are held in association with each other in a table.
The mobile station 200 measures each SIR, searches the mapping table for a CQI value corresponding to the measured SIR, and sends the value to the base station 100. Then, the base station 100 sends transmission data having a transport block size as large a size as possible (equal to or smaller than any of the transport block sizes acquired from the CQI mapping table), to the mobile station 200 through the HS-DSCH.
According to the above-described conventional techniques, the HS-SCCH is decoded so as to extract various parameters. Then, the HS-DSCH is demodulated and decoded using the extracted parameters. At this time, the HS-SCCH is convolution codes so as to be decoded. On the other hand, the HS-DSCH is turbo-encoded so as to be demodulated and decoded. This is because the data to be transmitted through the HS-SCCH has a relatively short bit length, and the data to be transmitted through the HS-DSCH has various bit lengths.
Since the data to be transmitted through the HS-DSCH has various bit lengths, the “ratio before and after the rate matching” changes. Therefore, either good or bad transmission (reception) characteristics may be realized (as described above, the lower the “ratio before and after the rate matching” is, the favorable reception characteristics may possibly be realized).
Since the data to be transmitted through the HS-SCCH has a constant bit length, the “ratio before and after the rate matching” does not change.
In such an environment that the “ratio before and after the rate matching” of the HS-DSCH is better (lower) than that of the HS-SCCH, the HS-DSCH can appropriately be received (i.e. the CRC result is “true” when the data is received and decoded). However, the HS-SCCH may not appropriately be received (i.e. the CRC result is “false” when the data is received and decoded). That is, generally, channels are so assigned that reception characteristics are better in the HS-SCCH than the case of the HS-DSCH. Depending on the encoding procedure of corresponding channel data or rate matching process, the good/bad reception characteristics of both of the HS-SCCH and HS-DSCH may become the other way around.
Even if the HS-DSCH, corresponding to the HS-SCCH whose CRC result is “false”, can appropriately be received, the receiving process for the HS-DSCH itself can not be carried out. This may result in a problem of deterioration in the transmission efficiency.